Use Turnstile by Cloudflare to perform captcha validation in your Rails app

Rails Form Builder supports Client-side validation using jQuery BootstrapValidator plugin. https://github.com/nghuuphuoc/bootstrapvalidator

Validate your ActiveRecord models on the client, showing their error and success messages by providing a validation controller.

BibleReferenceParser can parse scriptures passages, such as 'Gen. 1:15-18, 21' to the individual books, chapters and verses in the passage. It also provides validation for invalid book names, chapters, and verses.

A validation library for ActiveRecord models that allows introspection (User.name_is_required?) and supports database normalization (aka "form input cleaning").

Valcro is a validation library for arbitrary ruby objects. I's primary goal is to remain as simple as possible so that it can be useful to a large surface of use cases remaining easily extensible.

A Ruby API providing parsing, generation and validation of CAP(Common Alerting Protocol) messages.

A wrapper for backbone.validation in the Rails asset pipeline

DataValidator has almost active_recorde validation methods.

Access environment variables. Also includes presence validation, type coercion and default values.

Interpol is a toolkit for working with API endpoint definition files, giving you a stub app, a schema validation middleware, and browsable documentation.

This gem allows you to create use cases with ActiveModel validations and keep your model clean

ActiveModel email validation. Checks MX records, sub address, regex, whitelisted and blacklisted check

Combine email validation and transformations to produce canonical email addresses.

It provides validation of URIs (URLs) to ActiveModel records and ActionControllers in Rails 3.

A library for setting up model validations, such as in ActiveRecord.

Allows testing vor valid html (including html5) and css. In addition you can configure the gem to use a locally installed validation service instead of the online w3c servers

pure ruby PNG file manipulation & validation

CouchRest::ExtendedDocument provides aditional features to the standard CouchRest::Document class such as properties, view designs, callbacks, typecasting and validations.

Interface to the W3C Feed Validation online service http://validator.w3.org/feed/